Demonstration of two different types of beta1-receptors in man (selective blockade of the positively inotropic and the positively chronotropic effect of isoproterenol).

The hemodynamic effect of a beta-receptor blockade with mepindolol--a noncardioselective beta-receptor blocker--was studied in 6 male test subjects age 25 to 30 years with an increasing dose of isoproterenol. The cardiac output per min, the heart rate and the stroke volume, the wet blood pressure and the contractility parameters dp/dt max in the right and left ventricles were measured as part of the study. It was found that a dissociated right shift of the dose-effect curves occurred for the stroke volume, contractility parameters and heart rate. The following major conclusions can be made from an exact analysis of this result: 1. Evidence was produced that a distinction must be made in man with respect to the so-called beta1-receptors between those which mediate a specific effect on the heart rate and those which mediate a primarily positively inotropic effect. 2. There are apparently some beta-receptor blockers which inhibit the frequency receptors primarily and the inotropic receptors to a lesser extent. 3. Through their beta 2-effect, noncardioselective beta-receptor blockers can partly compensate for their negatively inotropic effect on the heart by maintaining the Frank Starling mechanism.